The Second Letter, written by the dragon, Frost, is a response to the letter written by Hoid in The Way of Kings. The letter reveals more about Hoid's motives and history as well as that of the Cosmere.
I'll address this letter to my old friend, as I have no idea what name you're using currently.
It has been confirmed by Brandon, through the husband of a reader (#46) on tor.com, that the recipient of the original letter is a dragon. Therefore, it can be reasonably inferred that the writer of this letter is that dragon (i.e., Frost).
Have you given up on the gemstone now that it is dead? And do you no longer hide behind the name of your old master? I am told that in your current incarnation you've taken a name that references what you presume to be one of your virtues.
Whether this was Tanavast's design or not, millennia have passed without Rayse taking the life of another of the sixteen. While I mourn for the great suffering Rayse has caused, I do not believe we could hope for a better outcome than this.
You, however, have never been a force for equilibrium. You tow chaos behind you like a corpse dragged by one leg through the snow. Please, harken to my plea. Leave that place and join me in my oath of nonintervention.